EP4302079A1

EP4302079A1 - Fibre optic chemical sensing

Info

Publication number: EP4302079A1
Application number: EP22709771.4A
Authority: EP
Inventors: Stephen Morgan; Liangliang LIU; Serhiy KORPOSH; Ricardo CORREIA
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2021-03-01
Filing date: 2022-03-01
Publication date: 2024-01-10
Also published as: WO2022185042A1; GB202102889D0

Abstract

The present application relates to a chemical sensor (100), comprising: an optical fibre (110); an optical cavity, optically coupled to the optical fibre (110), wherein the optical cavity comprises a dye with an absorbance and/or fluorescence spectrum that is responsive to a chemical concentration; a spectrometer (120) configured to determine a spectral response of the optical cavity (110); a processor (130) configured to determine a first chemical concentration from the dye absorbance and/or fluorescence spectrum by spectroscopy, and to determine a wavelength of one or more Fabry-Pérot interference fringes from the optical cavity (110). Also disclosed is a method of chemical sensing using a fibre optic sensor. Also disclosed is a method of fabricating a chemical sensor, comprising: coating, by sol-gel deposition, a distal end (117) of an optical fibre (110) with a silica matrix comprising dye; drying the silica matrix comprising dye in a dry inert gas. Aspects of the invention may be used in a healthcare context.

Description

FIBRE OPTIC CHEMICAL SENSING TECHNICAL FIELD The present invention relates to a method and apparatus for chemical sensing employing an optical fibre. BACKGROUND Chemical sensing (e.g. gas sensing) is often useful. One context in which chemical sensing is useful is in healthcare. Carbon dioxide (CO₂) in the human body is produced by cellular metabolism, and is transported in a dissolved format (H₂CO₃) via blood flow and diffuses into alveoli at the lungs where it is subsequently expelled from the body with each breath. In some pathological conditions such as asthma and chronic obstructive pulmonary disease, the functionality of the lungs is affected and, as a result, CO2 cannot be fully released through breathing. The retention of CO2 in the blood can cause respiratory acidosis which can be life-threatening. End-tidal CO2 (ETCO2) is the measurement of CO2 at the end of each breath and may be used to evaluate pulmonary diseases. ETCO2 may also be used to verify the correct placement of an endotracheal tube during the procedure of assisted ventilation. Humidity (i.e. a measurement of water vapour) is important in healthcare contexts. Humidity can affect the operation of expensive electrical equipment by enabling static discharge, and more importantly, can affect the comfort and health of the patient. Exhaled humidity in breath is also an important parameter that is measured in spirometry for diagnosis of pulmonary deficiencies, especially in the determination of pulmonary oxygen uptake during anaesthesia. Another example is in pH measurement. In the field of biomedicine, there is great demand for real-time and accurate pH measurement. For example, in vitro cell culture needs to be in the range of 6.5 to 7.7 as cell growth is diminished when the buffer pH is outside this range. For chronic wounds such as diabetic foot ulcers, monitoring changes in wound exudate pH can indicate healing rate or infection with early intervention helping to reduce economic costs. Active movement of tumour cells is an essential process of malignant proliferation, which will increase if the extracellular pH is moderately acidic (≈6.7). Foetal acidosis is commonly defined as a low umbilical pH, or a high umbilical base deficit. When defined by a low umbilical pH (with threshold for cut-off varying between 7.2 and 7.0), acidosis is associated with neonatal morbidity and mortality. Another example is in environmental measurement where fibre optic sensors are particularly favoured for sensing in hazardous environments such as coal mines and oil refineries where electrical sparks are dangerous in an explosive atmosphere. Ammonia detection is significant in safety monitoring of the workplace due to its toxicity to human health. For example there is a high risk of ammonia exposure in the livestock and fertiliser industries. Emission of Volatile Organic Compounds (VOCs) can be derived from a wide range of indoor and outdoor sources. Indoor sources include office supplies, household products, composite wood materials and furniture adhesive. Outdoor sources include chemical industries, food processing, vehicle manufacturers and transportation. The majority of VOCs have inimical effects on human health including headaches; nose, eye and throat irritation; and damage to the kidney and liver. A low-cost sensor that is compatible with the healthcare environment is desirable, and has the potential to make a positive contribution to improve health. Fibre optical sensors (FOS), due to the numerous advantages of light over electronic systems, have drawn a lot of research interest in a range of applications including healthcare¹. An interesting application is monitoring relevant parameters during MRI scanning where it is challenging to utilise conventional electrical sensors due to strong electromagnetic interference. Existing approaches for optical fibre sensing of CO2 use pH dyes such as thymol blue, phenol red and α-naphtholphthalein or fluorescent compounds such as 1-hydroxy-3,6,8- pyrenetrisulfonic acid trisodium salt (HPTS) with quaternary ammonium hydroxide which convert gaseous CO2 into a dissolved form, leading to a change of pH. Such pH dyes or fluorescent compounds change their spectral properties according to the degree of the pH change induced by CO2. The absorbing or fluorescent dyes are usually doped into a matrix film such as a silica or a polymer film along with the quaternary ammonium hydroxide on the optical fibre in a region where light interacts within the film such as the tip or cladding removed region. One common problem reported is cross-sensitivity to humidity, and the sensors are recommended to be applied in an environment with ¹ Correia, R., et al., Biomedical application of optical fibre sensors. Journal of Optics, 2018. 20(7): p. 073003. controlled humidity level, which is a major practical issue for the implementation of such sensors. An interesting optical fibre multi-sensing CO₂ sensor is reported by Wu et al. in which the dye-free polymer are in situ optically printed on the end surface of a multicore optical fibre, different polymers on the top of each core constitute an individual sensor for simultaneously sensing of CO₂ and temperature². Other interesting approaches of optical fibre multi-sensing include encapsulation of multiple fluorescent dyes on the fibre tip ³ , a two-film structure one the fibre tip ⁴ and an optofluidic laser on a microstructured optical fibre⁵. These approaches are relatively complex and at present are not compatible with simple, low cost manufacture and therefore widespread implementation. A potential candidate for optical fibre sensing of CO2 is to use pH indicators with organically modified silica (e.g. Ormosil) as the matrix film. Properties of the matrix film such as polarity and porosity vary based on the molar ratio of the mixtures, reaction temperature as well as drying condition and these can affect the sensing performance through the permeability of the CO2 as well as the cross-sensitivity to humidity. This makes relating the CO2 response difficult to interpret when humidity changes. A method of sensing CO2 with low RH cross talk is therefore highly desirable. More widely, fibre based spectroscopic measurements are applicable in a range of different contexts. An optical cavity comprising a dye that is coupled to an optical fibre can be likened to a mini-cuvette, in which spectroscopy can be performed to estimate the dye concentration, which can be related to external parameters. Accurate knowledge of the path length of the optical cavity is necessary to relate spectroscopic measurements of absorption in the cavity to external parameters. It may be difficult to control precisely the path length of an optical cavity e.g. due to variability in the manufacturing process. ² Wu, J., et al., In situ μ-printed optical fibre-tip CO2 sensor using a photocrosslinkable poly(ionic liquid). Sensors and Actuators B: Chemical, 2018. 259: p. 833-839. ³ Davenport, J.J., et al., Dual pO2/pCO2 fibre optic sensing film. Analyst, 2017. 142(10): p. 1711-1719. ⁴ Liu, L., et al., Multi-parameter optical fibre sensing of gaseous ammonia and _CO2. Journal of Lightwave Technology, 2019: p. 1-1. ⁵ Gong, C., et al., Distributed fibre optofluidic laser for chip-scale arrayed biochemical sensing. Lab on a Chip, 2018. 18(18): p. 2741-2748. Knowledge of the optical pathlength can remove the need for precise manufacture of the sensing region. SUMMARY According to a first aspect, there is provided a chemical sensor, comprising: an optical fibre; an optical cavity, optically coupled to the optical fibre, wherein the optical cavity comprises a dye with an absorbance and/or fluorescence spectrum that is responsive to a chemical concentration; a spectrometer configured to determine a spectral response of the optical cavity; a processor configured to determine a first chemical concentration from the dye absorbance and/or fluorescence spectrum by spectroscopy, and to determine a wavelength of one or more Fabry-Pérot interference fringes from the optical cavity. The chemical may be in the gas phase (or the liquid phase). The chemical sensor may be a gas sensor. The optical cavity may be disposed at a distal tip of the optical fibre. The optical cavity may be embedded in the optical fibre. The optical cavity may comprise a thin film. The spectrometer may be configured to determine a spectral response of the optical cavity in a reflection mode, or in a transmission mode. The processor may be configured to determine a second chemical concentration from the wavelength of the one or more Fabry-Pérot interference fringes. The second chemical concentration may comprise a humidity measurement, in which a concentration of water vapour is determined. The second chemical concentration may be determined in dependence of the wavelength of the first Fabry-Pérot interference fringe. The optical cavity may comprise more than one dye. The each of the dyes may have an absorbance and/or fluorescence spectrum that is responsive to a different chemical concentration (so that the concentration of more than one chemical can be determined by the processor from the spectral response of the optical cavity). The processor may be configured to correct the first chemical concentration using the wavelength of the one or more Fabry-Pérot interference fringes. The processor may be configured to determine an optical path length in the Fabry-Pérot cavity using the wavelength of the one or more Fabry-Pérot fringes. The first chemical concentration may comprise a concentration of CO₂. The first chemical concentration may comprise a concentration of ammonia (NH3). The optical cavity may comprise silica. The optical cavity may comprise a mesoporous sol-gel deposited silica matrix (e.g. formed from precursor comprising tetraethoxysilane, TEOS or methyltriethoxysilane, MTEOS). The dye may comprise at least one of thymol blue and tetramethylammonium hydroxide (TMAH). The dye may comprise tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS). For example, where the first chemical concentration comprises a concentration of ammonia, the dye may comprise TPPS. The optical cavity may have a thickness of between 2 microns and 10 microns. More preferably, the optical cavity may have a thickness of between 4 microns and 8 microns. The spectrometer may comprise a light source coupled to a proximal end of the optical fibre. The light source may comprise a broadband light source (e.g. comprising a tungsten filament). The light source may be configured to emit light with a spectral range comprising wavelengths of 400nm to 1000 nm. The spectrometer may have a spectral resolution lower than 2nm or lower than 1nm. The spectrometer preferably may have a spectral resolution that is lower than 0.5nm. According to a second aspect, there is provided a method of chemical sensing using a fibre optic sensor, comprising: illuminating an optical cavity at a distal end of an optical fibre with light coupled into a proximal end of the optical fibre, wherein the optical cavity comprises a dye with an absorbance and/or fluorescence spectrum that is responsive to a first chemical concentration; measuring a dye absorbance spectrum; determining a first chemical concentration from the dye absorbance spectrum by spectroscopy; and determining a wavelength of one or more Fabry-Pérot interference fringes from the optical cavity. The chemical may be in the gas phase (or the liquid phase). The chemical sensor may be a gas sensor. The optical cavity may be disposed at a distal tip of the optical fibre. The optical cavity may be embedded in the optical fibre. The spectrometer may be configured to determine a spectral response of the optical cavity in a reflection mode, or in a transmission mode. The method may comprise determining a second chemical concentration from the wavelength of the one or more Fabry-Pérot interference fringes. The second chemical concentration may comprise a humidity measurement, in which a concentration of water vapour is determined. The second chemical concentration may be determined in dependence of the wavelength of the first Fabry-Pérot interference fringe. The optical cavity may comprise more than one dye. Each of the dyes may have an absorbance spectrum that is responsive to a different chemical concentration (so that the concentration of more than one chemical can be determined by the processor from the spectral response of the optical cavity). The method may comprise correcting the first chemical concentration using the wavelength of the one or more Fabry-Pérot interference fringes. For example, correcting the first chemical concentration may comprise determining a thickness of the cavity, and correcting an absorbance to take account of the determined thickness of the cavity. The processor may be configured to determine an optical path length in the Fabry-Pérot cavity using the wavelength of the one or more Fabry-Pérot fringes. The first chemical concentration may comprise a concentration of CO₂. The first chemical concentration may comprise a concentration of ammonia (NH3). The optical cavity may comprise silica. The optical cavity may comprise a mesoporous sol-gel deposited silica matrix (e.g. formed from precursor comprising tetraethoxysilane. TEOS or methyltriethoxysilane, MTEOS). The dye may comprise at least one of thymol blue and tetramethylammonium hydroxide (TMAH). The dye may comprise tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS). For example, where the first chemical concentration comprises a concentration of ammonia, the dye may comprise TPPS. The optical cavity may have a thickness of between 2 microns and 10 microns. More preferably, the optical cavity may have a thickness of between 4 microns and 8 microns. The method may comprise illuminating the cavity using a light source coupled to a proximal end of the optical fibre. The light source may comprise a broadband light source (e.g. comprising a tungsten filament). The light source may emit light with a spectral range comprising 400nm to 800 nm wavelengths. The method may comprise using a spectrometer to determine a spectral response of the cavity. The spectrometer may have a spectral resolution of lower than 2nm or lower than 1nm, or lower than 0.5nm. According to a third aspect, there is provided a method of fabricating a chemical sensor, comprising: coating, by sol-gel deposition, a distal end of an optical fibre with a silica matrix comprising dye; drying the silica matrix comprising dye in a dry inert gas. The inventors have found that drying the silica matrix dramatically reduces permeability of the film to water molecules, which reduces sensitivity to humidity (thereby reducing cross-sensitivity of chemical sensing and humidity). The dry inert gas may comprise 100% nitrogen. The dry inert gas may have a water vapour content of less than 1%. The method of the third aspect may be used to produce a sensor for use in the first aspect, including any of the optional features described with reference to the first aspect. The features of each aspect (including optional features) may be combined with those of any other aspect. For example, features described with reference to the first aspect may be used in the method according to the second aspect. The chemical sensor according to the first aspect may be configured to perform the method according to the second aspect, including any optional features thereof. BRIEF DESCRIPTION OF THE DRAWINGS Example embodiments will be described, by way of example only, with reference to the drawings, in which: Figure 1 is a schematic diagram of a chemical sensor according to an embodiment; Figure 2 shows an optical fibre with a Fabry-Pérot cavity at the distal end thereof, with reflections from the interface between the optical fibre and the cavity, and from the interface between the cavity and air; Figure 3 shows a mesoporous film comprising dye; Figure 4 shows a spectral response of an example dye with changing CO₂ concentration; Figure 5 shows the correlation between absorbance values at two wavelengths for the example dye with different concentrations of CO₂; Figure 6 shows expressions for light reflected and transmitted at each interface; Figure 7 shows a simulated absorption spectra with a film thickness of 4 microns and a dye concentration of 2.7 mg/mL of thymol blue and TMAH; Figure 8 shows the difference between the Beer-Lambert spectrum and the spectrum that includes Fabry-Pérot interference fringes from Figure 7; Figure 9 shows the spectral response of a sensor cavity to CO2 for a film thickness of 4 micron and a dye concentration of 2.7 mg/mL; Figure 10 shows a simulated spectral response of the sensor cavity to CO2 for a film thickness of 3 microns and a dye concentration of 1.2 mg/mL; Figure 11 shows a simulated spectral response of sensor cavity to CO2 for a film thickness of 6 microns and a dye concentration of 1.2 mg/mL; Figure 12 compares the ratio of absorbance at 450 nm and 608 nm for different gas concentrations and thicknesses of cavity; Figure 13 shows experimental results obtained from a gas sensor according to an embodiment, comparing the response from different cavity thicknesses (obtained from stacking different numbers of successive depositions on the distal end of the fibre); Figure 14 shows an SEM micrograph of the distal end of the fibre, illustrating a measurement the film thickness of the cavity; Figure 15 shows the simulated absorption spectrum for each cavity length; Figure 16 shows the relationship between the wavelength of a selected interference fringe and refractive index of the cavity; Figure 17 shows a measured reflection spectrum obtained from a gas sensor according to an embodiment for i) 100% nitrogen and ii) 5.7% CO₂ (and 94.3% argon); Figure 18 shows a measured signal from a sensor according to an embodiment compared with a reference signal from a prior art device; Figure 19 shows a polynomial fit to the data obtained in Figure 18; Figure 20 shows measured spectra from a gas sensor according to an embodiment under different levels of humidity; Figure 21 shows average intensity values within a waveform (peak to peak) and a peak position (wavelength) under different relative humidity levels; Figures 22 and 23 shows a linear fits of the wavelength of a fringe peak against relative humidity; Figure 24 shows a comparison between an Fabry-Pérot fringe peak wavelength and the output of a prior art humidity sensor; Figure 25 shows CO2 sensitivity of a fibre optic sensor at different working temperatures; Figure 26 shows the variation in fringe peak wavelength for a selected Fabry-Pérot interference fringe at different CO₂ concentrations and different temperatures; Figures 27 and 28 show a comparison in response between a prior art logger and a gas sensor according to an embodiment for CO₂ concentration and humidity respectively; Figure 29 shows a test setup for evaluating embodiments; and Figure 30 shows a flow diagram of a method according to an embodiment; Figure 31a shows an example sensor in which a cavity is disposed between a first optical fibre and a second optical fibre; Figure 31b shows an example sensor in which a cavity is between two fused optical fibres; Figure 31c shows an example sensor in which a cavity is embedded within an optical fibre; Figure 32 shows an example spectral response from a fluorescent dye; Figure 33 shows an example spectral response from an optical cavity comprising a fluorescent dye with varying cavity thickness; Figure 34 shows an example spectral response from an optical cavity comprising a fluorescent dye with varying analyte concentration; Figure 35 shows the spectral response of a cavity comprising a dye comprising TPPS for different humidity levels; Figure 36 shows the wavelength shift of a selected absorption peak (D2) as different humidity levels are applied; Figure 37 shows sensitivity of wavelength changes for spectrographic features with varying humidity; Figure 38 shows the spectral response of a cavity comprising a dye comprising TPPS for different ammonia levels; Figure 39 shows the wavelength shift of a selected absorption peak (D1) as different ammonia levels are applied; and Figure 40 shows the sensitivity of the wavelength shift in the absorption peak (D1) with respect to ammonia concentration. DETAILED DESCRIPTION Figure 1 shows an example embodiment comprising: optical fibre 110, optical coupler 145, light source 140, spectrometer 120 and processor 130. The optical fibre 110 comprises a proximal end 116. The light source 140 is coupled to the proximal end 116 via the optical coupler 145, so that light from the light source propagates forwards within the optical fibre toward the distal end 117. An optical cavity is defined at the distal end 117, from which light is reflected to propagate backwards through the optical fibre 110 to be received at the spectrometer 120 via the optical coupler 145. The light source 140 may comprise a broadband light source, such as an incandescent lamp (e.g. tungsten, tungsten halogen etc). The spectrometer 120 is configured to determine a spectrum of the light reflected from the distal end 117 of the optical fibre 110. The spectrometer 120 outputs a signal that represents the spectrum to the processor 130. The processor 130 is configured to determine a concentration of one or more gases from the signal received from the spectrometer 120. In some examples the processor 130 and spectrometer 120 may be integrated. In other embodiments the processor 130 may be embodied in a computer that is separate from the spectrometer 120 (e.g. as show in Figure 29). The light source 140 and/or optical coupler 145 may similarly be incorporated into the spectrometer 120 (or may be separate therefrom, as depicted schematically in Figure 1. Figure 2 illustrates the distal end 117 of the optical fibre 110. As is conventional, the optical fibre 110 comprises a core 111 that forms a waveguide, within a cladding 112. The core 111 and cladding 112 have different refractive index, which confines light within the core 111. An extrinsic cavity 115 is provided on the tip of the distal end 117 of the optical fibre 110. The cavity 115 comprises a dye that is responsive to the concentration of a gas so that the absorbance spectrum of the cavity 117 changes with the concentration of the gas. In some embodiments, the dye may be sensitive to CO₂, for example, but other analytes (e.g. any chemical analyte capable of diffusing into the cavity) can also be sensed using a similar mechanism (by selecting an appropriate dye). For example, in some embodiments the dye may be sensitive to ammonia. The forward propagating light 150 from the broadband light source 140 is reflected from interfaces where there is a step change in refractive index. A first reflection 151 occurs at a first interface 113 between the core 111 and the material of the cavity 115. A second reflection 161 occurs at a second interface 114 between the material of the cavity 115 and air (for convenience the term “air” will be used herein, but it will be appreciated that sensors according to an embodiment may be deployed to sense gases in environments other than air). The spectrometer 140 will detect both these reflections. The cavity 115 is therefore configured as a Fabry-Pérot interferometer and interference fringes may be identified in the spectrum detected by the spectrometer 120. The inventors have appreciated that the Fabry-Pérot interference fringes can be usefully employed in the context of a gas sensor, as will be explained more fully below. In some embodiments, the Fabry-Pérot interference fringes may be used to determine a thickness of the optical cavity and thereby correct a dye response to a first gas concentration (e.g. CO2 or ammonia). In some embodiments, the Fabry-Pérot interference fringes may be used to identify changes in optical path length of the cavity, which may be associated with a second gas concentration (for example to determine relative humidity, which results in a change in refractive index of the cavity). In some embodiments, the Fabry- Pérot interference fringes may be used both to correct a measurement of a first gas concentration and to determine a concentration of a second gas by a different transduction mechanism. One way to produce a cavity 115 on the distal tip of the optical fibre 110 is to use a sol- gel dip coating process. An organically modified silica such as TEOS or MTEOS can be used as the sol-gel matrix precursor. The TEOS and MTEOS undergo hydrolysis and condensation during the Sol-gel reaction, resulting in a cross-linked silica network with the formation of siloxane bonds (≡Si-O-Si≡) between silanol groups (-Si-OH). The dye is consequently physically trapped inside the network in format of ion-pairs . An example method of producing a cavity 115 on the distal tip of the optical fibre 110 comprises first preparing a coating solution. Preparing the coating solution comprises: adding 2 ml of MTEOS into 6 ml of ethanol; dissolving 4 mg of TPPS into 1 ml of deionised water; mixing the MTEOS and TPPS solutions together and adding 2 µl of HCl (37%) while stirring and heating (to 50 °C) for one hour. The coating solution is then diluted with ethanol in a 50:50 volume ratio before dipping the distal tip of the optical fibre 110 in the diluted coating solution. The optical fibre 110 is then dried in ambient conditions for 48 hours. Figure 3 illustrates a matrix 200 comprising a silica 201 matrix and encapsulated dye 202 (e.g. thymol blue (HT) and TMAH (QOH), which may respectively be used as dye and phase transfer for sensing CO₂). Gas (e.g. CO2) diffuses into the film and reacts with the dye ion-pair producing a colour change. The film is preferably mesoporous, which enables the gas to diffuse readily and thereby reach the dye. The colour transmission of the above process for thymol Blue and TMAH can be explained by: The colour transits from red to blue as the OH- ion of phase transfer (TMAH) deprotonates the thymol blue forming ion pairs (Eq. (1)). While CO2 reacts with the hydrated water molecule of ion pairs, it produces protons that protonate the thymol blue (T-) leading to a colour change towards yellow (Eq. (2)). Figure 4 shows how the absorption spectrum of a film like that illustrated in Figure 3 changes with exposure to CO₂ at a range of different concentrations. With increasing CO₂ concentration, the absorbance at wavelength λ = 608 nm decreases and absorbance at λ = 450 nm increases. This is a result of a colour transition of the dye from blue to yellow on exposure to CO₂. The dye has two different states: protonated and deprotonated. These two states have different absorption features, and can be treated as two dyes which contribute to the absorption below and above the isosbestic point (λ = 500 nm), respectively. The absorbance value at a specific wavelength (A_λ) can be expressed as: Where is the film thickness; and are the concentration of deprotonated and protonated dye. is the concentration of the dye added during the film preparation and and ′ are the respective extinction coefficients of the two dyes, considered to be constant. As absorbance at the specific wavelength changes with CO2 concentration, the correlation between the concentration of deprotonated dye and CO₂ can be obtained via equation (1). The two extinction coefficients and can be obtained from: is the absorbance value corresponding with wavelength λ=450 nm when all dyes are in protonated status, that is is the absorbance value corresponding with wavelength λ=608 nm when all dyes are in deprotonated status, that is _{( )} Since the absorbance at each of the two wavelengths are linearly proportional to each other and are obtained at interception points with X and Y axis. Figure 5 shows a plot from which the absorbance values can be obtained, from which it was determined that . The thickness of the film defining the cavity 115 can be measured (e.g. by ellipsometry, SEM etc), thereby enabling calculation of as a function of CO2 concentration from Eq. (8) as shown in Figure 4. Consequently, the CO₂ response of an optical fibre CO₂ sensor according to an embodiment can be simulated (as shown in Figure 4) provided the thickness of the film on the tip of the optical fibre is known. Due to variations in the fabrication process, the thickness of the film on the tip of the fibre cannot be predicted accurately. This is a problem for practical sensors based on dye spectroscopy (e.g. based on an absorbance at a characteristic wavelength or range of characteristic wavelengths, or based on a ratio of absorbance at two characteristic wavelengths, or two ranges of characteristic wavelengths). The cavity on the tip of the optical fibre 110 forms an extrinsic Fabry-Pérot cavity 115. The cavity 115 has interference fringes in its reflection spectrum due to interference of reflections from the cavity. These fringes allow the calculation of film thickness by using the following equation: Where ^ is the cavity length (i.e. thickness of the film defining the cavity), n is the refractive index of the film and λ₁ and λ₁ are the central wavelengths of two adjacent interference peaks. The cavity may have a length of few microns (e.g. 6 microns or more generally, between 2 and 10 microns). The light transmits from the fibre 110 into the film that defines the cavity 115 and from the cavity 115 into air; reflectance occurs at each interface (differential refractive index medium) on its pathway. Figure 6 provides expressions for the light at each stage of propagating through the cavity 115. The electric field associated with the forward propagating light field 150 is given by . The first reflection 151 is given by where R₁ is the reflectance associated with the first interface 113 between the optical fibre core 111 and the cavity 115 and δ is 1 or 0 respectively as determined by the half-wave loss or none on interface 1 or 2. The reflected light will experience a 180 degree phase change when it reflects from a medium of higher refractive index (δ = 1) and no phase change when it reflects from a medium of smaller index (δ = 0). The electric field of the forward propagating light 152 in the cavity 115 immediately following the first interface 113 is given by where A₁ is the transmission loss associated with the first interface 113. The forward propagating light 153 incident at the second interface 114 is given by where a is the absorption coefficient of the material of the cavity 115, ( ) is the diffracted electric field after path length , with being the cavity thickness (i.e. the distance between first interface 113 and second interface 114). The electric field of the reflected light 161 from the second interface 114 is given by where is the reflectance associated with the second interface 114 between the cavity 115 and air. The electric field of the backward propagating light 162 reflected from the second interface 114 arriving at the first interface 113 is . The electric field of the backward propagating light 162 from this reflection that is transmitted through the first interface 113 to return within the fibre core 111 is given by . The electric field coupled back into the fibre 110 arising from both reflections can therefore be written as (the sum fields from light 151 and 161): The normalised intensity coupled back into the optical fibre 110 can be expressed as Due to losses within the cavity from absorption, and the relatively low reflectivity from interfaces (e.g. due to surface roughness), higher order reflections (from multiple passes through the cavity) are treated as negligible. Some of the expressions in (9) can be obtained from: Where r is the radial coordinate, is the power normalising coefficient, is the beam-waist diameter, the beam radius is given by , z is the propagation distance from the beam waist at , and , and ^ are the refractive index of the fibre core, cavity and air, respectively. The normalised intensity may be written as: The total loss factor ^ may written as: The absorption a may be written as Where is the Rayleigh range and is the Guoy phase shift. Figure 7 shows a validation of a spectrum obtained from the above equations for a tip- based optical fibre sensor. A spectrum 222 is shown according to the above equations (including the Fabry-Pérot fringes) along with a spectrum 221 obtained from the Beer- Lambert law in accordance with equation (3). Figure 7 shows that the result obtained using the Fabry-Pérot interferometry equations matches that from the Beer-Lambert law, but includes the interference fringes that are characteristic of Fabry-Pérot interferometry. When a perturbation is introduced to the cavity, such as a change of refractive index, the phase difference changes in proportion to the optical path length of the interferometer. The change of phase difference is observed as a shift of the wavelength of the interference fringes in the reflection spectrum, and the degree of the perturbation can be quantitatively related to the shift in wavelength. In the case of a silca matrix, an increase in the atmospheric relative humidity will cause water molecules to be adsorbed by the unreacted hydrophilic silanol group (-Si-OH) of the silica matrix via hydrogen bonds (illustrated in Figure 3), leading to a change of refractive index. As a result, the phase difference of the two interference beams of the Fabry-Pérot interferometer is changed and the interference fringes undergo a shift. Embodiments are not limited to the specific mechanism of changing refractive index in response to relative humidity. There are other mechanisms by which the optical path length of a cavity layer may be purturbed by changes in relative humidity. For example, many polymeric materials swell in response to humidity. A polymer layer (e.g. a porous polymer layer such as a polymer with intrinsic microporosity) may be used as the cavity, for example. Figure 8 shows the result of subtracting spectrum 221 from spectrum 222. The result is the interference oscillation which has an average magnitude of zero. The fringe contrast (defined as the magnitude between the peak and valley) in the subtracted absorption spectrum is bigger at the dye absorption peak (λ=608 nm) than in wavelength regions with lower absorption. This is because the total loss factor ^ is smaller where absorption is lower, which results in a low fringe contrast in normalised intensity, or the transmission value. Since the absorption value is reciprocal to the transmission value, the fringe contrast is larger when absorption is higher. The spectral response of a detector according to an embodiment can be determined from equations (3) to (17) for different concentrations of gas (e.g. CO₂). Figure 9 shows an example with a cavity thickness of 4 microns and a dye concentration of 2.7 mg/mL. Figure 10 shows an example with a cavity thickness of 3 microns and a dye concentration of 1.2 mg/mL, and Figure 11 shows an example with a cavity thickness of 6 microns and a dye concentration of 1.2 mg/mL. As already shown in Figure 5, the absorbance ratio varies with CO₂ gas concentration (for the example dye), so can be considered independent of the dye concentration and film thickness. Figure 12 illustrates that this ratio remains substantially constant at different film thickness The refractive index of the sol-gel film (encapsulating thymol blue and TMAH) used as the cavity of the example fibre optic sensor was characterised by depositing a layer on a silicon wafer via dip coating and measuring the film properties using a spectroscopic ellipsometer (after drying in nitrogen). A similar process could be used to characterise a film comprising a different dye (e.g. sensitive to a different gas). Figure 13 shows a measured spectral response for a fibre sensor according to an embodiment with different thicknesses of cavity, each formed from a different number of coating cycles. The absorption window, corresponding to the absorption band of thymol blue, appears between 500 to 700 nm and the Fabry-Pérot interferometer fringes appear in each spectrum. The absorption value increases with increasing number of coating cycles due to the increased optical pathlength according to the Beer-Lambert law. The number of fringes increases after each layer coating as a result of the increase of cavity length (thickness of the film). The visibility of fringes decreases at the fourth layer, and the fringes are even smaller after further increase of the number of coating cycles. The absorption value between 500 to 700 nm decreases after drying for 24 hours due to decomposition of base catalysts during the evolution of the sol-gel film. One coating cycle on the tip of the fibre has a relatively low absorbance after drying as shown in Figure 13, so may be less suitable for CO₂ measurement due to reduced colour change. Multiple coating cycles provide thicker films with more encapsulated dye, resulting in a higher absorbance values and more measurable colour change after exposure to CO₂. Three layers are used in the example embodiments described below, since this provides a useful balance between visibility of Fabry-Pérot fringes and discernable color changes (for colorimetric analysis of CO₂ concentration). The refractive index of the sensing film after drying was measured as 1.501 ± 0.02 via ellipsometry. The thickness for a cavity produced using three coating cycles was determined as 5.83 ± 0.09 μm from Eq. (5) which agrees well with the 5.95 μm measured with SEM in Figure 14. Figure 14 shows an example fibre 110 with a coated film cavity 115 at the distal end 117 characterised by SEM. A Fabry-Pérot interferometric sensor simulation was built using the measured cavity parameters (i.e. film RI and thickness) using the equations disclosed above. The extinction coefficient of the dye at deprotonated status was obtained with UV-Visible absorption spectroscopy. Figure 15 shows absorption spectra of the optical fibre Fabry- Pérot interferometric sensor with different cavity lengths ranging from 1 micron to 8 microns. The number of fringes increases with increasing cavity length and absorbance values dye absorption wavelengths also increases with thickness. The spectral changes with increase of cavity length agree with experimental results shown in Figure 13. For a cavity with length 6 microns, the wavelength of a selected interference fringe is proportional to the refractive index, with a coefficient of proportionality of 386 nm/RIU (at an initial fringe wavelength around 585 nm). Figure 16 shows a linear fit to the (peak) wavelength of the selected fringe with respect to refractive index of the cavity. Fig. 17 illustrates the reflection spectrum from an embodiment in 100 % N₂ and 5.7 % CO₂. Within the absorption window (λ= 350-670 nm), the normalised intensity value increases around 600 nm and decreases at 450 nm after exposure to CO2 as a result of protonation of the dye. Using thymol blue dye, the colour of the film turns from blue (deprotonated status) towards yellow (protonated status). The position of the interference fringes remains unchanged during the test indicating no cross-talk between the fringe position and the concentration of CO2. This also suggests that there is no detectable refractive index change after the interaction of CO2. The average intensity value of the selected wavelengths (596 - 617 nm, wavelengths between two adjacent interference peaks around the absorption peak) was extracted to compare with the CO2 reading from the prior art datalogger as shown in Figure 18. This range avoids any intensity change at a single wavelength induced by the shift of Fabry-Pérot fringes resulting from changes in relative humidity. The average intensity is more insensitive to any shifts in Fabry-Pérot interference fringes. The normalised intensity value (on the y axis in Figure 18) is related to the change of CO2 concentration by a polynomial relationship and Figure 18 therefore demonstrates the responsivity and reversibility of the FOS, rather than a quantitative agreement on concentration. Figure 19 shows the relationship between the normalised intensity and the CO2 concentration from the prior art datalogger. Hysteresis is observed for the reverse trace which is likely caused by the incomplete release of CO2 during the recording time. The response time and recovery time is calculated as 98 s and 418s, respectively. The relative humidity level during the CO₂ test was relatively stable at 19 ± 1.9 %. Figure 20 to 24 show the results of testing detection of humidity using the example fibre optic sensor. In this testing the concentration of CO₂ is under the limit of detection of the commercial datalogger (< 20 ppm), so the test environment can be treated as CO₂ free. The interference fringes from the example sensor undergo a redshift after exposure to increasing levels of RH as illustrated in the reflection spectrum in Figure 20. This is due to increased film refractive index as demonstrated in Figure 16 .The intensity value remains relatively stable as the RH level increases from 0 % to ~80 % whereas a selected peak wavelength (around 686 nm) exhibits a gradual increase (illustrated in Figure 21). The intensity slightly increases as the concentration increases from 0 % to 65 %, due to the protonation of water molecules to alkalic thymol blue. The overall increase of the intensity caused by the 65 % of RH would correspond (without correction) to a CO₂ concentration of 232 ppm (based on the CO₂ response in Figure 19. The small drop of the intensity when RH is above 70 % is caused by the decrease of reflectivity of light due to adsorption of water molecules on the film-air interface. The cross sensitivity of CO2 concentration to RH can be ignored as negligible in most circumstances (and can alternatively be corrected, for example using a calculation based on data like that shown in Figure 21). Each peak in the interference fringes shows the same trend of linear correlation to the RH level range from 0 % up to ~80 %. The interference fringe peak at around 686 nm exhibits the highest sensitivity of 0.19 nm/1% RH. Therefore, the wavelength with initial position of 686 nm is tracked to indicate the RH change, for comparison with the reading from the reference (prior art) datalogger. The wavelength of this peak, as a function of RH, is shown in Figures 22 and 23 (obtained from the set-ups shown in Figure 29). These two curves cover the whole range of RH. Although the RH setting on the environmental chamber 320 was set up to 100 %, the true RH appears to saturate at around 90 % only. The wavelength shift of the FOS shows good agreement with the RH change inside the environmental chamber. Figure 24 shows a repeatability test of a fibre optic sensor according to an embodiment when varying RH from 50 % up to 90 % and the wavelength of the FOS exhibits excellent reversibility and repeatability to RH. The response and reverse time of the sensor for the RH from ambient level (~ 33 %) to 76 % were calculated as 32 s and 56 s, respectively. The faster response time of RH than CO2 may be attributed to the faster diffusion speed of water molecule in the silica film due to its smaller molecular weight according to Knudsen diffusion. Figures 25 and 26 show cross sensitivity to temperature. Figure 25 shows the correlation of intensity to different levels of CO₂ at different temperatures from 20 °C to 40 °C. This shows that the example sensor has a lower sensitivity at higher temperature. This is more pronounced at higher concentrations (> 0.5 %). The intensity decreases approximately 6 % when temperature increases from 20°C to 40°C at 0.03 % of CO₂ whereas it drops about 9 % at 5 % of CO₂. Within 5°C of temperature change, the percent error for measuring 5 % CO₂ is < 14 %. The wavelength of the Fabry-Pérot interference fringes, on the other hand, does not significantly change as the CO₂ concentration increases, as shown in Figure 26 (the biggest change is ~ 0.3 nm). Figures 27 and 28 illustrate the use of a fibre optic sensor according to an embodiment for measurement of CO₂ and relative humidity from human breath. Figures 27 and 28 respectively show CO₂ and relative humidity measurements taken by the reference datalogger and example embodiment. There is good agreement between the fibre optic sensor according to an embodiment and the prior art reference datalogger, with a percentage error of around 3% for CO2 and 2.2% for relative humidity. Figure 29 shows a test setup for demonstrating performance of embodiments. The test setup comprises an environmental test chamber 320, a prior art CO2, temperature and humidity logger 360, temperature controlled heating mat 313, temperature controller 311, gas source 350, flow meter 330, water flask 340, voltage supply 312 and gas sensor 100. The gas sensor 100 is arranged with the distal end 117 in the chamber 320, in thermal contact with the heating mat 313. The heating mat 313 is controlled by the temperature controller 311, and is arranged to modulate the temperature of the Fabry-Pérot dye sensor at the distal end 117 of the optical fibre 110. The gas sensor is the same as shown in Figure 1, with like features being provided with like reference numerals in Figure 29. The description of these features with reference to Figure 1 is equally applicable here. The environmental chamber 320 is configured to provide a stabilised temperature of 25 degrees C, and a relative humidity range of 50% to 100%. The concentration of CO₂ in the chamber 320 may be varied by controlling the flow of CO₂ from the gas source 350 based on the output of the flow meter 330. The gas source 350 comprises a source of nitrogen and a source of CO₂. Breath sample measurement was also undertaken, in which a fibre optic sensor according to an embodiment was placed with a prior art sensor assembly in a Tedlar bag, and then sampling breath into the Tedlar bag. A heat moisture exchanger was used behind the mouthpiece to prevent condensation of humidity after the exhaled gases leave the human body (to avoid the sudden temperature drop from 37 degrees C to ~22 degrees C). For calibration of CO₂, the measurement chamber shown in Figure 29 was filled up with 100% nitrogen and then the concentration modified step-by-step by regulating the CO₂ flow and allowing each step to stabilise for at least 5 min. The reflection spectra of the gas sensor according to an embodiment and readings from the prior art datalogger were recorded simultaneously. The average normalised intensity value of the wavelengths between 596 nm to 617 nm (peak-to-peak), corresponding to the absorption band of the thymol blue, was used as the optical signal for calibration with the prior art datalogger. The CO2 calibration curve was obtained by averaging the optical signal during the stabilised period (the last 5 mins of each step) against the average concentration of CO2 from the datalogger during the same period. The reported percentage error represents the difference between the measurement value of a measurand by using the gas sensor according to an embodiment and the prior art reference sensor, divided by the reference value reported from the reference sensor. Gas sensor according to embodiments may exhibit comparable performance to prior art CO2 dataloggers for determining CO2 and RH level from a breath sample, but with much smaller sensor size. Sensors according to an embodiment provide for the smallest possible transducer capable of measuring both CO2 and RH. One advantage of the compact size of sensors according to embodiments is application with confined space and/or sealed environments. A sensor according to an embodiment may be incorporated into a needle gas sensing device that can sample through a self-sealing layer (e.g. a self- sealing polymer) without breaking a seal. A sensor according to an embodiment may be easily incorporated into a ventilation system for supporting endotracheal intubation by detecting CO2 expelled from lungs and monitoring the RH delivered to the patient from a ventilation system in a clinical intensive care unit. More generally, sensors according to embodiments may be used to monitor carbon dioxide and relative humidity in breath or breath samples in a medical context. Figure 30 illustrates a method according to an embodiment, in which, at step 410, an optical cavity at a distal end of an optical fibre is illuminated. The optical cavity comprises a dye with an absorbance spectrum that is responsive to a first gas concentration. At step 420 a spectrum of light reflected by the cavity is measured. At step 430, a first gas concentration is determined from the absorbance of light by the dye of the cavity (i.e. using spectroscopy). At step 440, a wavelength of one or more Fabry- Pérot interference fringes is determined. The wavelength of the one or more fringes may be used to correct the first gas concentration (e.g. by correcting for a thickness of the cavity), or may be used to determine a second gas concentration (e.g. in the case that the cavity changes optical path length in response to the second gas concentration). The example shown in Figure 2 shows a single fibre, with a cavity that is interrogated in reflection mode, but this is not essential, and in certain embodiments the cavity may be interrogated in a transmission mode. The cavity in the embodiments shown in Figures 31a to 31c may be similar to the cavity described with reference to the foregoing embodiments, but interrogated in transmission mode (as an alternative or additional option to interrogating the optical cavity in reflection mode). Figure 31a shows a first optical fibre, similar to that shown in Figure 2, comprising a cavity 115 disposed on the distal tip of the first optical fibre. A second optical fibre is configured to receive light transmitted through the cavity. The light transmitted by the cavity will include Fabry-Pérot fringes, similar to reflected light. Figure 31b shows a first optical fibre and a second optical fibre that are spliced together via the cavity 115. Light may be transmitted to the cavity 115 via the first optical fibre, and the transmission characteristics of the cavity determined by a spectrometer coupled to the second optical fibre. Figure 31c shows a single optical fibre in which a cavity 115 has been embedded. The cavity 115 can thereby be interrogated in transmission and/or reflection. Figure 32 shows an absorption spectrum 501 for a fluorescent dye, and a corresponding fluorescence spectrum 502. Light is absorbed by the dye and emitted with a Stokes shift as fluorescence. For simplicity, the quantum yield is 1 and the spectra are both normalised to 1. The example fluorophore is ATT0390, ATTO-Tec GmbH, but any suitable fluorescent dye can be used. Figure 33 shows the spectral response of a cavity comprising the dye of Figure 32 for different cavity length (i.e. different film thickness). The excitation light interrogating the optical cavity has a spectrum that matches the absorption of the dye shown in Figure 32. The peak 511 corresponds with reflected or transmitted light after the excitation beam has traversed the optical cavity. The peak 512 corresponds with fluorescent light, emitted by the cavity in response to absorption of the excitation light. Fringes are visible in the reflected or transmitted excitation light, as already discussed with reference to a non-fluorescent dye. The strength of the fluorescence increases with increasing film thickness (with more of the excitation light being absorbed and re-emitted as fluorescent light with thicker films). The positions (i.e. wavelength) of fringes observed in the excitation peak can be used to determine the thickness of the film. Figure 34 shows the spectral response of a cavity comprising the dye of Figure 32 for different concentrations of fluorophore (dye). The excitation light interrogating the optical cavity again has a spectrum that matches the absorption of the dye shown in Figure 32. The peak 511 corresponds with reflected or transmitted light after the excitation beam has traversed the optical cavity. The peak 512 corresponds with fluorescent light, emitted by the cavity in response to absorption of the excitation light. Fringes are visible in the reflected or transmitted excitation light, as already discussed with reference to a non-fluorescent dye. The strength of the fluorescence peak 512 increases with increasing fluorophore concentration (with more of the excitation light being absorbed and re-emitted with higher concentrations of fluorophore). Having already determined the thickness of the cavity from the positions of the fringes in the excitation peak 511, the concentration of the fluorophore can be determined from the relative intensity of the excitation and fluorescence peaks 511,512. Embodiments thereby enable quantitative measurements to be performed based on dyes comprising fluorophores without precise control over cavity/film thickness or fluorophore concentration within the film/cavity. Figure 35 shows the spectral response of a cavity comprising a dye comprising TPPS, which is sensitive to ammonia, for different humidity levels. The absorption peaks labelled D1 and D3 represent absorption peaks of the TPPS dye that are responsive to ammonia (centred on ~490nm and ~705nm respectively). The absorption peak D2 and transmission peak P1 result from Fabry-Pérot interference. D2 is a local minima in the intensity of light reflected from the fibre tip between the absorption peaks D1 and D3, and P1 is a local maximum in reflection in the wavelength range 750nm to 850nm. The wavelength (corresponding with minimum intensity) for both D1 and D3 are largely unchanged in response to different humidity levels. The value of the local minimum at D1 is insensitive to humidity. The value of the local minimum at D3 shows some sensitivity to humidity as a result of the Fabry-Pérot fringes overlaying the dye absorption peak. The wavelength of D2 and P1 is sensitive to humidity, because these features of the spectrum correspond with Fabry-Pérot fringes (and the cavity changes its optical length in response to changes in humidity). Figure 36 shows the wavelength shift of the D2 absorbance peak at the different humidity levels represented in Figure 35 as the fibre tip is exposed to the different humidity levels. Figure 37 shows wavelengths of D1, D2, D3 and P1 features (shown in Figure 35) for the different humidity levels represented in Figure 35. Figure 37 shows that D2 and P1 have a substantially linear sensitivity to humidity, and that D1 and D3 are not sensitive to humidity. Figure 38 shows the spectral response of a cavity comprising a dye comprising TPPS for different ammonia levels. The same features D1, D2, D3 and P1 can be seen in the spectra. D1 and D3 are sensitive to the varying concentrations of ammonia, and shift in both wavelength and intensity. The is more clearly illustrated in Figure 39, which shows the wavelength variation in D1 over time in response to different concentrations of ammonia. The wavelength of D1 tracks the ammonia concentration. Figure 40 shows the sensitivity of the wavelength of D1 with respect to ammonia concentration. Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. The examples provided in the detailed description are intended to provide examples of the invention, not to limit its scope, which should be determined with reference to the accompanying claims.

Claims

CLAIMS 1. A chemical sensor, comprising: an optical fibre; an optical cavity, optically coupled to the optical fibre, wherein the optical cavity comprises a dye with an absorbance and/or fluorescence spectrum that is responsive to a chemical concentration; a spectrometer configured to determine a spectral response of the optical cavity; a processor configured to determine a first chemical concentration from the dye absorbance spectrum by spectroscopy, and to determine a wavelength of one or more Fabry-Pérot interference fringes from the optical cavity.

2. The chemical sensor of claim 1, wherein the processor is configured to determine a second chemical concentration from the wavelength of the one or more Fabry-Pérot interference fringes.

3. The chemical sensor of claim 2, wherein the second chemical concentration comprises a humidity measurement.

4. The chemical sensor of claim 2 or 3, wherein the second chemical concentration is be determined in dependence of the wavelength of the first Fabry-Pérot interference fringe.

5. The chemical sensor of any preceding claim, wherein the processor is configured to correct the first chemical concentration using the wavelength of the one or more Fabry-Pérot interference fringes.

6. The chemical sensor of claim 5, wherein correcting the first chemical concentration comprises using the wavelength of the one or more Fabry-Pérot interference fringes.

7. The chemical sensor of any preceding claim, wherein the first chemical concentration comprises a concentration of CO₂ or NH₃.

8. The chemical sensor of any preceding claim, wherein the optical cavity comprises silica.

9. The chemical sensor of any preceding claim, wherein the dye comprises at least one of: thymol blue; tetramethylammonium hydroxide, TMAH; and tetraphenylporphyrin tetrasulfonic acid hydrate, TPPS.

10. The chemical sensor of any preceding claim, wherein the optical cavity has a thickness of between 2 microns and 10 microns.

11. The chemical sensor of any preceding claim, wherein the spectrometer comprises a broadband light source coupled to a proximal end of the optical fibre.

12. The chemical sensor of claim 11, wherein the light source is configured to emit light with a spectral range comprising wavelength of 400nm to 1000 nm.

13. The chemical sensor of any preceding claim, wherein: the optical cavity comprises more than one dye, each of the dyes having an absorbance spectrum that is responsive to a different chemical concentration, and the processor is configured to determine the concentration of more than one chemical from the spectral response of the cavity.

14. A method of chemical sensing using a fibre optic sensor, comprising: illuminating an optical cavity that is optically coupled to an optical fibre with light coupled into a proximal end of the optical fibre, wherein the optical cavity comprises a dye with an absorbance and/or fluorescence spectrum that is responsive to a first chemical concentration; measuring a spectrum of light reflected by the optical cavity; determining the first chemical concentration from the spectrum by spectroscopy; and determining a wavelength of one or more Fabry-Pérot interference fringes from the optical cavity.

15. The method of claim 14, further comprising determining a second chemical concentration from the wavelength of the one or more Fabry-Pérot interference fringes.

16. The method of claim 15, wherein the second chemical concentration comprises a humidity measurement.

17. The method of claim 15 or 16, wherein the second chemical concentration is determined in dependence of the wavelength of the first Fabry-Pérot interference fringe.

18. The method of any of claims 14 to 17, further comprising correcting the first chemical concentration using the wavelength of the one or more Fabry-Pérot interference fringes.

19. The method of any of claims 14 to 18, wherein the first chemical concentration comprises a concentration of CO2 or NH3.

20. The method of any of claims 14 to 20, wherein the optical cavity comprises a mesoporous sol-gel deposited silica matrix.

21. The method of any of claims 14 to 20, wherein the dye comprises at least one of: thymol blue; tetramethylammonium hydroxide, TMAH; and tetraphenylporphyrin tetrasulfonic acid hydrate, TPPS.

22. The method of any of claims 14 to 21, wherein the optical cavity has a thickness of between 2 microns and 10 microns.

23. The method of any of claims 14 to 22, comprising illuminating the cavity using a broadband light source coupled to a proximal end of the optical fibre.

24. The method of any of claims 14 to 23, comprising using a spectrometer to determine a spectral response of the cavity.

25. A method of fabricating a chemical sensor, comprising: coating, by sol-gel deposition, a distal end of an optical fibre with a silica matrix comprising dye; drying the silica matrix comprising dye in a dry inert gas.